Referenced from Overlawyered.com
The first shipment of the new [SmarTrip card] machines did not have the audio and Braille features required under the Americans with Disabilities Act. But Metro thought it could roll out the machines and add the audio and Braille a couple of months later. When disability advocates raised concerns, Metro realized that going forward would violate the ADA, and the transit agency halted the rollout.
So nearly three weeks after every station was to have its own SmarTrip card dispenser, riders at nearly half of the stations in the Metrorail system are out of luck if they need to buy a card.
Read it here: New Metro Card Machine not compliant
The principle being that, if it is offered to one it must be offered to all.